WARNING
If you use a Multimeter to check the connections on the Presence Detection Pins: Do not touch other pins than these 5 shown below !The memory module might get damaged from the testing voltage of your Multimeter.

Also obey the usual rules for antistatic device handling !

Most newer IBM PS/2 machines use the 72-pin Single Inline Memory Modules (SIMM) on either planar or memory expansion cards.
Very often there are more than one of these 72-pin connectors available and one might wish to upgrade the memory with adding another memory module.
But: the machine comes up with a nice 215-error code and the error-code list says it were "Check memory, it might be wrong type or wrong speed".

"How can the system know that ?"

Pretty simple: There are 4 pins reserved on the module, which are called 'Presence Detect Pins' - and which tell the systemboard

This is an accepted coding for the 4MB/80nS-module P/N 79F3988 / 79F1000 (FRU 79F1003) used on the L40SX (8543), see the Selfmade Memory Page as well.

This is an accepted coding for the 8MB/80nS-module P/N 79F3989 (FRU 79F1004) used on the L40SX (8543), see the Selfmade Memory Page as well. However: It must be a single-sided module with chips that have the typecode -LJP8 after the sizecode !

This is an accepted coding for the 2MB/80nS-module P/N 79F3987 (FRU 79F1002) used on the L40SX (8543), see the Selfmade Memory Page as well.